The present invention relates to an apparatus and a method for printing a printing paste such as a cream solder or a paste used for formation of thick film circuits to obtain printed circuit boards or the like for the mounting of electronic components.
A screen printing apparatus referred to above has been used, for example, in a printing process when circuits are to be mounted with electronic components. With the recent tendency towards more compact electronic devices, finer and sophisticated circuit boards are required, consequently inviting the need for the cream solder to be printed with high accuracy.
A conventional example of a screen printing method will be described with reference to the drawings.
FIG. 4 is a schematic diagram to explain a conventionally-employed general screen printing apparatus and a method executed by the apparatus. FIG. 5 indicates a state where a printed circuit pattern is attached to a conventional general screen plate.
In the screen printing apparatus in FIG. 4, a printing paste 3 such as a cream solder or the like is supplied onto a screen printing plate 2 where a circuit pattern is formed. A positioning stage 4 for securely positioning an object to be printed such as a circuit board 1 to be printed is moved up and down immediately below the screen printing plate 2. A first printing squeegee 5 which is lowered to the face of the screen printing plate 2 in its right direction's movement for printing and a second printing squeegee 6 which is lowered to the face of the screen printing plate 2 in its left direction's movement for printing are moved up and down above the screen is printing plate 2. When one of the squeegees 5 and 6 is moved down, the squeegee moves in the horizontal direction keeping touch with the screen printing plate 2.
In general, the circuit pattern is arranged in a manner as shown in FIG. 5. That is, the circuit pattern in the longitudinal direction is set approximately in the same direction as the printing direction P by the squeegees 5 and 6, or in a direction approximately orthogonal to the printing direction P. In FIG. 5, 7a is a circuit pattern portion extending approximately in parallel to the printing direction P, and 7b is a circuit pattern portion extending approximately in the orthogonal direction to the printing direction P. Printed circuit portions 8a, 8b of the same configuration (referring to FIGS. 6A-6D) are respectively formed for the circuit pattern portions 7a, 7b by the printing action.
The conventional screen printing method is carried out in the following fashion.
After the to-be-printed object 1 is positioned and fixed by the positioning stage 4, the stage 4 with the object 1 is moved up to immediately below the screen printing plate 2. Thereafter, the first printing squeegee 5 is lowered to print the printing paste 3 to the object 1 while moving in the horizontal direction. Then, the positioning stage 4 is descended, thus completing one printing process. The same process is conducted in the same manner as above by the second squeegee 6. The printing process is repeatedly performed afterwards.
According to the conventional arrangement as above, when the printed portion 8b formed approximately in the orthogonal direction to the printing direction is compared with the printed portion 8a formed approximately along the printing direction, the printing thickness and the printing width are not uniform, that is, the portion 8b is printed irregularly and not accurately. FIGS. 6A-6D show the states of the printed portions by the printing paste, e.g., cream solder, more specifically, FIGS. 6A and 6B indicating the printing states of the printed portion 8a of one lead of an electronic component such as a quad flat pack (QFP) extended approximately in parallel to the printing direction and FIGS. 6C and 6D representing the printing states of the printed portion 8b extended approximately in the orthogonal direction to the printing direction.